Microgeomorphology And Leaching Mineralization Of The Permian Super-large Bauxite Deposit In Dazhuyuan,Northern Guizhou

Permo-Carboniferous karstic bauxites are substantial among all bauxite deposits throughout the whole world,so as to China especially to the Guihzou province.For this type of bauxite,research of 1)mechanism of sedimentary environments and geomorphological features under paleokarsts controlling distributions and changes of thickness of ore bodies and bauxite ore type;2)hydrological system model controlling bauxitization and 3)leaching processes during formation of bauxite deposits influencing the horizontion of ore-bearing rock series,the mobilization and concentration of major,trace and rare earth elements and the ore grade are usually hampered by the lack of systematic detailed data collection during exploration engineering on a large scale and the limit of amount of samples tested in previous studies.Dazhuyuan Mining District is located in the northeast part within the Wuchuan–Zheng'an–Daozheng(Wu–Zheng–Dao)area in which a super-large deposit of bauxite,with the feature of magnitude,continuation and high-grade,was proven by the 106 Geological Brigade,Guizhou Bureau of Geology and Mineral Exploration and Development.This study is supported by 2 research projects of Guizhou Bureau of Geology and Mineral Exploration and Development and data obtained in the assignment of exploration managed by the author.This study used data containing the total thickness,the thickness of four types of bauxite ore,bauxite ore grade of the Dazhuyuan Formation bauxite deposit in the DMD and so on from 329 drillcores within the Dazhuyuan Mining District(DMD)to generate high-precision geomorphological reconstructions for the period of bauxite formation.Here,we undertook a petrological,mineralogical,and geochemical study to analyze the leaching process and bauxitization with the lithofacies features(96 petrographic thin sections,17 sets of test result of XRD and 20 sets of test result of SEM),concentration and mobilization of major,trace and rare earth elements(99 samples,4563 sets of data).The contour maps of thickness of ore-bearing rock series and ore bodies indicate that the thick bauxite deposits are controlled by the geomorphology of area in which limestones of Huanglong Formation formed.Continuously high-grade bauxite ores are located within depressions or in gently sloping areas between depressions and elevated erosional regions.By the contrast to typically bauxite deposits in Zunyi,controll of lithology of basement and the thickness of underlying carbonate rocks to the geomorphological features in the early Permian is the key facter of the formation of bauxite deposits in the DMD.The particular hydro-system formed within ore-bearing rock series and the underlying limestones of Huanglong Formation and shales of Hanchiatien under exposure and hot-wet climate,with the leaching process of atmospheric precipitation proceeding.On the other hand,good drainage provided by the water-permeable underlying units(Huanglong Formation)contributes to the generation of high-grade bauxite ores.The lithological association and differentiation of minerals from the bottom to the top presented features of weathering and leaching process through the profile,so as to high values of chemical index of alteration(CIA)of samples of bauxite layers.The concentrations of Al ascended from the bottom to the top through the profile,with the Fe and Si reducing.The higher content of Ti within the upper member of Dazhuyuan Formation indicated that mobile elements lost and immobile elements enriched during leaching process.With the elevation of groundwater table,bauxitic claystones formed and the vadose bauxite switched to the phreatic bauxite.Contrast to the average value of crust,rare earth elements are depleted in most of bauxite samples and enriched in bauxitic claystones,which indicate REEs may be hosted in the claystones.On the other hand,REEs are enriched to the lower member.HREEs which are hosted in the Ti-bearing minerals(rutile,anatase,and leukoxen)remained easily during weathering and leaching processes.Rare element(Be)and elements of iron group(Co,Ni,Cr)ascended from the top to the bottom of the weathering profile.Trace elements such as Ba?Sr?Rb?As?Cs and Sc are enriched within layers under the bauxite layers.Li?Th?U?Cu?Co?Hf?Sn?Nb?Ta?Zr and W are concentrated within the ore layers,especially for the Li.From the upper member to the lower member,Mo,Bi and V reduced,while Be?Cr and Ni ascended.The formation of bauxite deposits within the DMD is a long and fluctuant period.Exposure,weathering and leaching process provided abundant parent materials for bauxitization,while paleogeographic environment and paleo-geomorphology of sedimentary basements controlled the thickness of ore-bearing rock series,distribution of ore bodies and bauxite ore type,with the leaching process during and after the bauxitization controlling the ore grade.